Precise control of vertical transistor gate length

ABSTRACT

Transistor and methods of forming the same include forming a channel fin on a bottom source/drain region. A dielectric fill is formed around the channel fin with a gap in an area directly above the channel fin that has a width greater than a width of the channel fin. Spacers are formed in the gap. The dielectric fill is etched away. A gate stack is formed on sidewalls of the channel fin directly underneath the spacers.

BACKGROUND

Technical Field

The present invention generally relates to semiconductor devices and,more particularly, to the use of an L-shaped spacer for decreasingvariations in vertical transistor gate length.

Description of the Related Art

Vertical transistors employ channel regions that are oriented generallyperpendicular to the plane of an underlying substrate—as opposed toconventional transistors which generally have channel regions that are apart of, or are otherwise parallel to, the underlying substrate. Thisorientation has significant potential for device scaling, moretransistors can fit within a given chip surface area.

However, one challenge in forming a vertical transistor is incontrolling the device's gate length. The gate length determines variousproperties of the final device, and conventional fabrication processesresult in significant variation in gate length from one device to thenext.

SUMMARY

A method for forming a transistor includes forming a channel fin on abottom source/drain region. A dielectric fill is formed around thechannel fin with a gap in an area directly above the channel fin thathas a width greater than a width of the channel fin. Spacers are formedin the gap. The dielectric fill is etched away. A gate stack is formedon sidewalls of the channel fin directly underneath the spacers.

A method for forming a transistor includes etching a substrate around amask to form a first fin and a bottom source/drain region. A bottomspacer is formed directly on the bottom source/drain region, around thefirst fin. A channel fin is formed on the bottom source/drain region.The first fin is laterally etched to form the channel fin, such that themask overhangs sidewalls of the channel fin. A dielectric fill is formedaround the channel fin with a gap in an area directly above the channelfin that has a width greater than a width of the channel fin. Spacershaving an L-shaped cross-section are formed in the gap. Forming theL-shaped spacers includes forming a dielectric bilayer film in the gapwith a bottom film and a top film, etching away horizontal regions ofthe dielectric bilayer film, and etching away remnant portions of thetop film, leaving behind the spacers. A top source/drain region isformed directly on a top surface of the channel fin, between thespacers. The dielectric fill is etched away. A gate stack is formed onsidewalls of the channel fin directly underneath the spacers.

A transistor includes a vertical channel fin directly on a bottomsource/drain region. A gate stack is formed on sidewalls of the verticalchannel fin. A pair of spacers is formed directly above the gate stack,one above each sidewall of the vertical channel fin. A top source/drainregion formed directly on a top surface of the vertical channel fin,between the spacers.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a cross-sectional diagram of a step in the formation of atransistor in accordance with the present principles;

FIG. 2 is a cross-sectional diagram of a step in the formation of atransistor in accordance with the present principles;

FIG. 3 is a cross-sectional diagram of a step in the formation of atransistor in accordance with the present principles;

FIG. 4 is a cross-sectional diagram of a step in the formation of atransistor in accordance with the present principles;

FIG. 5 is a cross-sectional diagram of a step in the formation of atransistor in accordance with the present principles;

FIG. 6 is a cross-sectional diagram of a step in the formation of atransistor in accordance with the present principles;

FIG. 7 is a cross-sectional diagram of a step in the formation of atransistor in accordance with the present principles;

FIG. 8 is a cross-sectional diagram of a step in the formation of atransistor in accordance with the present principles;

FIG. 9 is a cross-sectional diagram of a step in the formation of atransistor in accordance with the present principles;

FIG. 10 is a cross-sectional diagram of a step in the formation of atransistor in accordance with the present principles;

FIG. 11 is a cross-sectional diagram of a step in the formation of atransistor in accordance with the present principles;

FIG. 12 is a cross-sectional diagram of a step in the formation of atransistor in accordance with the present principles;

FIG. 13 is a cross-sectional diagram of a step in the formation of atransistor in accordance with the present principles; and

FIG. 14 is a block/flow diagram of a method of forming a transistor inaccordance with the present principles.

DETAILED DESCRIPTION

Embodiments of the present invention use a spacer that is formed on topof a vertical transistor's channel region with overhang to protect agate region from subsequent etch and polish steps. Such etch and polishsteps would otherwise cause unpredictability in the gate length. In oneparticular embodiment, the spacer is formed with an L-shaped crosssection.

Referring now to FIG. 1, a step in forming a vertical transistor isshown. A semiconductor fin 104 is formed from a semiconductor substrate102 using a hardmask 106 and an anisotropic etch, such as reactive ionetching (RIE). Although it is contemplated that the semiconductor fin104 may be formed from the same material as the semiconductor substrate102, it should be understood that different materials may be used.

In one embodiment, the semiconductor substrate 102 is formed from abulk, silicon-containing material. Illustrative examples ofsilicon-containing materials suitable for the bulk-semiconductorsubstrate include, but are not limited to, silicon, silicon germanium,silicon carbide, silicon germanium carbide, polysilicon, epitaxialsilicon, amorphous silicon, and multi-layers thereof. Although siliconis the predominantly used semiconductor material in wafer fabrication,alternative semiconductor materials can be employed, such as, but notlimited to, germanium, gallium arsenide, gallium nitride, cadmiumtelluride, and zinc selenide. Although not depicted herein, thesemiconductor substrate 102 may also be a semiconductor on insulator(SOI) substrate.

The semiconductor fin 104 may therefore be any appropriate semiconductorchannel material. In addition to the possibilities set forth above, thesemiconductor fin 104 may be formed from a III-V compound semiconductormaterial. The term “III-V compound semiconductor” denotes asemiconductor material that includes at least one element from Group IIIof the Periodic Table of Elements (International Union of Pure andApplied Chemistry (IUPAC) Group 13) and at least one element from GroupV of the Periodic Table of Elements (IUPAC Group 15). Typically, theIII-V compound semiconductors are binary, ternary or quaternary alloysincluding III/V elements. Examples of III-V compound semiconductors thatcan be used in the present invention include, but are not limited toalloys of gallium arsenic, aluminum arsenic, indium gallium arsenic,indium aluminum arsenic, indium aluminum arsenic antimony, indiumaluminum arsenic phosphorus, indium gallium arsenic phosphorus andcombinations thereof.

A hardmask 106 is used to define the fin 104, with material outside thatcovered by the hardmask 106 being etched away as shown. It isspecifically contemplated that this etch may be performed as part of ashallow trench isolation (STI) process, but it should be understood thatother forms of anisotropic etching may be employed instead. As usedherein, an “anisotropic etch process” denotes a material removal processin which the etch rate in the direction normal to the surface to beetched is greater than in the direction parallel to the surface to beetched.

STI regions are formed by etching a trench in the substrate utilizing aconventional dry etching process such as RIE or plasma etching. RIE is aform of plasma etching in which during etching the surface to be etchedis placed on a radio-frequency powered electrode. Moreover, during RIEthe surface to be etched takes on a potential that accelerates theetching species extracted from plasma toward the surface, in which thechemical etching reaction is taking place in the direction normal to thesurface. Other examples of anisotropic etching that can be used at thispoint of the present invention include ion beam etching, plasma etchingor laser ablation. Alternatively, fin structure 104 can be formed byspacer imaging transfer processes.

While it is specifically contemplated that the hardmask 106 may beformed from silicon nitride, it should be understood that alternativehardmask materials may include silicon oxides, silicon oxynitrides,silicon carbides, silicon carbonitrides, etc. Spin-on dielectrics mayalso be utilized as a hardmask material including, but not limited to:silsequioxanes, siloxanes, and boron phosphate silicate glass (BPSG).The hardmask 106 may be patterned using photolithographic processes,where the hardmask 106 may be produced by applying a photoresist layerto the semiconductor surface, exposing the photoresist layer to apattern of radiation, and then developing the pattern into thephotoresist layer utilizing conventional resist developer.

It should be noted that the substrate region 102 underneath the fin 104operates as the bottom source/drain region for the device and will bereferred to equivalently herein as either the “substrate” or the “bottomsource/drain region.” The substrate region 102 is therefore doped inaccordance with the conductivity type of the device and may be in situdoped or may alternatively be doped by bombardment with a dopantspecies. As used herein, the term “conductivity type” denotes a dopantregion being p-type or n-type. As used herein, “p-type” refers to theaddition of impurities to an intrinsic semiconductor that createsdeficiencies of valence electrons. In a silicon-containing substrate,examples of p-type dopants, i.e., impurities, include but are notlimited to: boron, aluminum, gallium and indium. As used herein,“n-type” refers to the addition of impurities that contributes freeelectrons to an intrinsic semiconductor. In a silicon containingsubstrate examples of n-type dopants, i.e., impurities, include but arenot limited to antimony, arsenic and phosphorous.

In a first embodiment, dopant species may be implanted vertically. Thehardmask 106 prevents dopant ions from entering the fin 104, and anoptional sacrificial spacer may be deposited along the sidewalls of thefin 104 to further insulate the fin 104 from implantation. A thermalanneal is then used to activate the dopant. In a second embodiment, asacrificial spacer may be used before a source/drain region recess isperformed, followed by epitaxial growth of source/drain regions with insitu doping. In a third embodiment, the substrate 102 may be grown within situ doping prior to patterning the fin 104. In this thirdembodiment, a doped epitaxial growth is used to form the layer thatbecomes substrate 102, followed by an undoped epitaxial growth step toform the layer that will later become the fin 104. The depth of thebottom source/drain region may be about 10 nm to about 50 nm deep.

Referring now to FIG. 2, a step in forming a vertical transistor isshown. A bottom spacer 202 is deposited over the horizontal surfacesusing a directional deposition such as, e.g., high-density plasma CVD(HDPCVD) or physical vapor deposition (PVD). It is specificallycontemplated that silicon nitride, silicon oxide or a combination ofsilicon oxide and silicon nitride may be used as the bottom spacer, butany appropriate dielectric material may be used in its place. The bottomspacer 202 is used to insulate the bottom source/drain region of thesubstrate 102 from the gate conductor that will be deposited later.

The fin 104 is laterally etched, narrowing to form channel fin 204. Thelateral trim of fin 104 may be performed using an isotropic etch such asan isotropic silicon RIE that selectively removes the channel materialof the fin 104 and does not affect silicon nitride, silicon dioxide, orother dielectrics that may be present.

Although HDPCVD is specifically contemplated, it should be understoodthat physical vapor deposition (PVC) or gas cluster ion beam (GCIB)deposition may be used instead. In embodiments that use PVD, asputtering apparatus may include direct-current diode systems, radiofrequency sputtering, magnetron sputtering, or ionized metal plasmasputtering. In embodiments that use GCIB deposition, a high-pressure gasis allowed to expand in a vacuum, subsequently condensing into clusters.The clusters can be ionized and directed onto a surface, providing ahighly anisotropic deposition.

Referring now to FIG. 3, a step in forming a vertical transistor isshown. A dielectric fill layer 302 is deposited using, e.g., chemicalvapor deposition (CVD). The dielectric material is deposited to a heightabove that of the hardmask 106 and subsequently polished down using,e.g., a chemical mechanical planarization (CMP) process that stops onthe hardmask material. It is specifically contemplated that silicondioxide may be used for the dielectric fill layer 302, but it should beunderstood that any appropriate material may be used instead.

It should be understood that, although CVD is specifically contemplatedfor deposition of the dielectric fill material, PVD, ALD, or GCIBdeposition may be used instead. CVD is a deposition process in which adeposited species is formed as a result of chemical reaction betweengaseous reactants at greater than room temperature (e.g., from about 25°C. about 900° C.). The solid product of the reaction is deposited on thesurface on which a film, coating, or layer of the solid product is to beformed. Variations of CVD processes include, but are not limited to,Atmospheric Pressure CVD (APCVD), Low Pressure CVD (LPCVD), PlasmaEnhanced CVD (PECVD), and Metal-Organic CVD (MOCVD) and combinationsthereof may also be employed.

CMP is performed using, e.g., a chemical or granular slurry andmechanical force to gradually remove upper layers of the device. Theslurry may be formulated to be unable to dissolve, for example, thehardmask material, resulting in the CMP process's inability to proceedany farther than that layer.

Referring now to FIG. 4, a step in forming a vertical transistor isshown. The hardmask 106 is etched away using any appropriate etchingprocess including, e.g., a wet etch or dry etch. The removal of thehardmask 106 leaves behind a gap 402 that exposes the top of fin 204.

Referring now to FIG. 5, a step in forming a vertical transistor isshown. A first film 502 is conformally deposited in the gap 402 using,e.g., a low-k dielectric or silicon nitride, although it should beunderstood that any appropriate material may be used instead. A low-kdielectric is a material having a dielectric constant k that is lowerthan that of silicon dioxide. A second film 504 is deposited conformallyover the first film 502 and may be formed from, e.g., silicon dioxide,though it should be understood that any appropriate material may be usedinstead.

Referring now to FIG. 6, a step in forming a vertical transistor isshown. The first and second films 502 and 504 are etched using a timedanisotropic etching process to remove material from exposed horizontalsurfaces. This etch leaves spacers 602 having an L-shaped cross sectionas well as remnants 604 of the second film. A portion of the top surfaceof the fin 204 is exposed between the spacers 602.

Referring now to FIG. 7, a step in forming a vertical transistor isshown. The remnants 604 of the second film are removed using anyappropriate etch. It is specifically contemplated that a wet or dry etchis used that is selective to the remnants 604. As used herein, the term“selective” in reference to a material removal process denotes that therate of material removal for a first material is greater than the rateof removal for at least another material of the structure to which thematerial removal process is being applied. The spacers 602 and the fin204 therefore remain undamaged by removal of the remnants 604.

The exposed portion of the fin 204 may be doped by ion bombardment.Optional implantation of a shallow layer of the top portion of thechannel fin 204 is performed to improve the electrical junction with thesubsequently formed top source/drain region.

Referring now to FIG. 8, a step in forming a vertical transistor isshown. A top source/drain region 802 is formed on the fin 204, betweenthe spacers 602. It is specifically contemplated that the topsource/drain region 802 may be epitaxially grown from the fin 204. Thetop source/drain region 802 may be doped in situ or via an implantationprocess.

The terms “epitaxial growth and/or deposition” means the growth of asemiconductor material on a deposition surface of a semiconductormaterial, in which the semiconductor material being grown hassubstantially the same crystalline characteristics as the semiconductormaterial of the deposition surface. The term “epitaxial material”denotes a material that is formed using epitaxial growth. In someembodiments, when the chemical reactants are controlled and the systemparameters set correctly, the depositing atoms arrive at the depositionsurface with sufficient energy to move around on the surface and orientthemselves to the crystal arrangement of the atoms of the depositionsurface. Thus, in some examples, an epitaxial film deposited on a {100}crystal surface will take on a {100} orientation.

In some embodiments, in which the epitaxial semiconductor material ofthe top source/drain region 802 is composed of silicon, the silicon gassource for epitaxial deposition may be selected from the groupconsisting of hexachlorodisilane, tetrachlorosilane, dichlorosilane,trichlorosilane, methylsilane, dimethylsilane, ethylsilane,methyldisilane, dimethyldisilane, hexamethyldisilane, and combinationsthereof. In some embodiments, in which the epitaxial semiconductormaterial of the top source/drain region 802 is composed of germanium,the germanium gas source for epitaxial deposition may be selected fromthe group consisting of germane, digermane, halogermane,dichlorogermane, trichlorogermane, tetrachlorogermane, and combinationsthereof. In some embodiments, in which the epitaxial semiconductormaterial of the top source/drain region 802 is composed of silicongermanium, the silicon sources for epitaxial deposition may be selectedfrom the group consisting of silane, disilane, trisilane, tetrasilane,hexachlorodisilane, tetrachlorosilane, dichlorosilane, trichlorosilane,methylsilane, dimethylsilane, ethylsilane, methyldisilane,dimethyldisilane, hexamethyldisilane, and combinations thereof, and thegermanium gas sources may be selected from the group consisting ofgermane, digermane, halogermane, dichlorogermane, trichlorogermane,tetrachlorogermane and combinations thereof. The germanium content ofthe epitaxial layer of silicon germanium may range from 5% to 70%, byatomic weight %. In another embodiment, the germanium content of theepitaxial layer of silicon germanium may range from 10% to 40%.

Referring now to FIG. 9, a step in forming a vertical transistor isshown. The oxide fill 302 is selectively etched away, exposing theunderlying spacer 202 and the sidewalls of the vertical channel fin 204.

Referring now to FIG. 10, a step in forming a vertical transistor isshown. A gate dielectric layer 1002 is conformally formed on thevertical channel fin 204. It is specifically contemplated that the gatedielectric layer 1002 may be formed from a high-k dielectric, which is amaterial having a dielectric constant k that is higher than that ofsilicon dioxide. A gate 1004 is deposited over the gate dielectric layer1002. It is specifically contemplated that the gate 1004 may be formedfrom a work function metal layer. The gate dielectric layer 1002 and thegate 1004 together form a gate stack and may include any other layersappropriate to the stack.

It is specifically contemplated that the gate dielectric layer 1002 maybe formed from an oxide of hafnium, but it should be understood that anyappropriate high-k dielectric may be used, with examples includingoxides of tantalum, zirconium, or aluminum.

In the case of a p-type device, the work function metal layereffectuates a p-type threshold voltage shift. In one embodiment, thework function of the p-type work function metal layer ranges from 4.9 eVto 5.2 eV. As used herein, “threshold voltage” is the lowest attainablegate voltage that will turn on a semiconductor device, e.g., transistor,by making the channel of the device conductive. The term “p-typethreshold voltage shift” as used herein means a shift in the Fermienergy of a p-type semiconductor device towards a valence band ofsilicon in the silicon containing material of the p-type semiconductordevice. A “valence band” is the highest range of electron energies whereelectrons are normally present at absolute zero.

In one embodiment, the p-type work function metal layer may be composedof titanium and its nitride/carbide. In one specific embodiment, thep-type work function metal layer is composed of titanium nitride. Thep-type work function metal layer may also be composed of titaniumaluminum nitride, ruthenium, platinum, molybdenum, cobalt, and alloysand combinations thereof.

In an n-type device, the work function metal layer effectuates an n-typethreshold voltage shift. As used herein, an “n-type work function metallayer” is a metal layer that effectuates an n-type threshold voltageshift. “N-type threshold voltage shift” as used herein means a shift inthe Fermi energy of an n-type semiconductor device towards a conductionband of silicon in a silicon-containing material of the n-typesemiconductor device. The “conduction band” is the lowest lying electronenergy band of the doped material that is not completely filled withelectrons. In one embodiment, the work function of the n-type workfunction metal layer ranges from 4.1 eV to 4.3 eV. In one embodiment,the n-type work function metal layer is composed of at least one oftitanium aluminum, tantalum nitride, titanium nitride, hafnium nitride,hafnium silicon, or combinations thereof.

It is specifically contemplated that the gate dielectric layer 1002 andthe gate 1004 are deposited using an isotropic deposition process suchas CVD. Excess material is subsequently anisotropically etched away fromany surface not covered by the spacers 602.

Referring now to FIG. 11, a step in forming a vertical transistor isshown. A conductor 1102 is filled in around the transistor device. It isspecifically contemplated that tungsten may be used as the conductor1102, but any appropriate conductive material may be used instead. Theconductor 1102 may be deposited using any appropriate deposition processincluding one of, e.g., CVD, PVD, ALD, or GCIB deposition, andsubsequently polished down to the level of the top of the spacers 602and the top source/drain region 802 using, e.g., CMP.

Referring now to FIG. 12, a step in forming a vertical transistor isshown. The conductor 1102 is anisotropically etched back to the level ofthe gate dielectric 1002. The etch back process is generally difficultto control with precision. The spacers 602 protect the gate dielectric1002 and the gate 1004 from the etch back, such that the length of thesefeatures can be precisely controlled and repeatably produced. Theremaining conductor 1202 forms an electrical contact to the gate 1004.

Referring now to FIG. 13, a step in forming a vertical transistor isshown. A passivating layer 1302 is deposited conformally over theconductor 1202 and the top source/drain region 802. It is specificallycontemplated that the passivating layer 1302 may be formed from siliconnitride, but it should be understood that any appropriate dielectricmaterial may be used instead.

It is to be understood that the present invention will be described interms of a given illustrative architecture; however, otherarchitectures, structures, substrate materials and process features andsteps may be varied within the scope of the present invention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

The present embodiments may include a design for an integrated circuitchip, which may be created in a graphical computer programming language,and stored in a computer storage medium (such as a disk, tape, physicalhard drive, or virtual hard drive such as in a storage access network).If the designer does not fabricate chips or the photolithographic masksused to fabricate chips, the designer may transmit the resulting designby physical means (e.g., by providing a copy of the storage mediumstoring the design) or electronically (e.g., through the Internet) tosuch entities, directly or indirectly. The stored design is thenconverted into the appropriate format (e.g., GDSII) for the fabricationof photolithographic masks, which typically include multiple copies ofthe chip design in question that are to be formed on a wafer. Thephotolithographic masks are utilized to define areas of the wafer(and/or the layers thereon) to be etched or otherwise processed.

Methods as described herein may be used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present principles, as well as other variations thereof, means thata particular feature, structure, characteristic, and so forth describedin connection with the embodiment is included in at least one embodimentof the present principles. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This may be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

Referring now to FIG. 14, a method for fabricating a vertical transistoris shown. Block 1402 forms a mask 106 on a layer of semiconductormaterial. As noted above, the mask defines a fin region, and block 1404etches the fin 104 from the semiconductor material, leaving bottomsource/drain region 102. It should be noted that the semiconductormaterial may be doped prior to formation of the fin 104 or,alternatively, the bottom source/drain region 102 may be doped byimplantation after forming the fin 104.

Block 1406 deposits the bottom spacer layer 202 on the bottomsource/drain region 102 around the fin 104. The fin 104 is thenlaterally etched to form trimmed channel fin 204 in block 1408. Adielectric fill 302 is deposited around the channel fin 204 and the mask106 in block 1410. The dielectric fill 302 is then polished down to theheight of the mask 106 in block 1412 and the mask 106 is etched away inblock 1414.

Block 1416 conformally deposits the spacer bilayer in the gap 402 leftwhen the mask 106 was etched away. The spacer bilayer includes the firstfilm 502 and the second film 504. Block 1418 then etches away thehorizontal sections of the spacer bilayer using an anisotropic etch andblock. Block 1420 implants ions in the fin 204 as described above.

Block 1422 etches away the sacrificial remnants 604 of the second spacerfilm 504, leaving behind the spacers 602, which have an L-shapedcross-section. Block 1424 then epitaxially grows the top source/drainregion 802 from the top surface of the channel fin 204. It isspecifically contemplated that the top source/drain region 802 may be insitu doped during the growth process, but in an alternative embodimentthe top source/drain region may be doped by an implantation process.

Block 1426 etches away the dielectric fill to expose the sidewalls ofthe channel fin 204. Block 1428 deposits the gate dielectric layer 1002using a conformal deposition process such as, e.g., CVD. Block 1430 thenforms the gate 1004 from an appropriate material such as a work functionmetal. Any leftover material from the formation of the gate dielectriclayer 1002 and the gate 1004 are then etched away, leaving a gate ofprecise length under the spacers 602. Block 1432 deposits a gate contactconductor material such as tungsten and polishes and etches the gatecontact conductor material down to the top level of the gate dielectriclayer 1002. It should be noted that the presence of the spacers 602protect the underlying gate 1004 and gate dielectric layer 1002 from thepolish and etch back of the gate conductor 1202. Block 1434 thendeposits a passivating dielectric layer 1302 over the gate conductor1202 and the top source/drain region 802, encapsulating the device.

Having described preferred embodiments of a system and method (which areintended to be illustrative and not limiting), it is noted thatmodifications and variations can be made by persons skilled in the artin light of the above teachings. It is therefore to be understood thatchanges may be made in the particular embodiments disclosed which arewithin the scope of the invention as outlined by the appended claims.Having thus described aspects of the invention, with the details andparticularity required by the patent laws, what is claimed and desiredprotected by Letters Patent is set forth in the appended claims.

What is claimed is:
 1. A method for forming a transistor, comprising:forming a channel fin on a bottom source/drain region; forming adielectric fill around the channel fin with a gap in an area directlyabove the channel fin that has a width greater than a width of thechannel fin; forming spacers in the gap; etching away the dielectricfill; and forming a gate stack on sidewalls of the channel fin directlyunderneath the spacers.
 2. The method of claim 1, wherein the spacershave an L-shaped cross-section.
 3. The method of claim 2, whereinforming the spacers comprises conformally forming a dielectric bilayerfilm in the gap.
 4. The method of claim 3, wherein forming the spacersfurther comprises etching away horizontal regions of the dielectricbilayer film.
 5. The method of claim 4, wherein forming the spacerscomprises etching away a remnant portion of a top film of the dielectricbilayer film, leaving behind the spacers.
 6. The method of claim 3,wherein the dielectric bilayer film comprises a bottom film of siliconnitride and a top film of silicon dioxide.
 7. The method of claim 1,further comprising forming a top source/drain region directly on a topsurface of the channel fin, between the spacers.
 8. The method of claim1, wherein forming the channel fin comprises etching a substrate arounda mask to form a first fin.
 9. The method of claim 8, further comprisingforming a bottom spacer directly on the bottom source/drain regionaround the first fin.
 10. The method of claim 9, wherein forming thechannel fin further comprises laterally etching the first fin to formthe channel fin, such that the mask overhangs sidewalls of the channelfin.
 11. The method of claim 8, further comprising etching away the maskafter deposition of dielectric fill material to form the gap.
 12. Amethod for forming a transistor, comprising: etching a substrate arounda mask to form a first fin and a bottom source/drain region; forming abottom spacer directly on the bottom source/drain region, around thefirst fin; forming a channel fin on the bottom source/drain region;laterally etching the first fin to form the channel fin, such that themask overhangs sidewalls of the channel fin; forming a dielectric fillaround the channel fin with a gap in an area directly above the channelfin that has a width greater than a width of the channel fin; formingspacers having an L-shaped cross-section in the gap, comprising: forminga dielectric bilayer film in the gap comprising a bottom film and a topfilm; etching away horizontal regions of the dielectric bilayer film;and etching away remnant portions of the top film, leaving behind thespacers; forming a top source/drain region directly on a top surface ofthe channel fin, between the spacers; etching away the dielectric fill;and forming a gate stack on sidewalls of the channel fin directlyunderneath the spacers.
 13. The method of claim 12, further comprisingetching away the mask after deposition of dielectric fill material toform the gap.
 14. The method of claim 12, wherein the bottom filmcomprises silicon nitride and the top film comprises silicon dioxide.